Re: [PATCH v1 3/5] dt-bindings: Add DT bindings for NVIDIA Tegra AHB DMA controller

[Stephen Warren <swarren@wwwdotorg.org>]

On 10/03/2017 04:32 AM, Jon Hunter wrote:
> 
> 
> On 03/10/17 00:02, Dmitry Osipenko wrote:
>> On 02.10.2017 20:05, Stephen Warren wrote:
>>> On 09/29/2017 09:11 PM, Dmitry Osipenko wrote:
>>>> On 29.09.2017 22:30, Stephen Warren wrote:
>>>>> On 09/27/2017 02:34 AM, Jon Hunter wrote:
>>>>>>
>>>>>> On 27/09/17 02:57, Dmitry Osipenko wrote:
>>>>>>> On 26.09.2017 17:50, Jon Hunter wrote:
>>>>>>>>
>>>>>>>> On 26/09/17 00:22, Dmitry Osipenko wrote:
>>>>>>>>> Document DT bindings for NVIDIA Tegra AHB DMA controller that presents
>>>>>>>>> on Tegra20/30 SoC's.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
>>>>>>>>> ---
>>>>>>>>>     .../bindings/dma/nvidia,tegra20-ahbdma.txt         | 23
>>>>>>>>> ++++++++++++++++++++++
>>>>>>>>>     1 file changed, 23 insertions(+)
>>>>>>>>>     create mode 100644
>>>>>>>>> Documentation/devicetree/bindings/dma/nvidia,tegra20-ahbdma.txt
>>>>>>>>>
>>>>>>>>> diff --git
>>>>>>>>> a/Documentation/devicetree/bindings/dma/nvidia,tegra20-ahbdma.txt
>>>>>>>>> b/Documentation/devicetree/bindings/dma/nvidia,tegra20-ahbdma.txt
>>>>>>>>> new file mode 100644
>>>>>>>>> index 000000000000..2af9aa76ae11
>>>>>>>>> --- /dev/null
>>>>>>>>> +++ b/Documentation/devicetree/bindings/dma/nvidia,tegra20-ahbdma.txt
>>>>>>>>> @@ -0,0 +1,23 @@
>>>>>>>>> +* NVIDIA Tegra AHB DMA controller
>>>>>>>>> +
>>>>>>>>> +Required properties:
>>>>>>>>> +- compatible:    Must be "nvidia,tegra20-ahbdma"
>>>>>>>>> +- reg:        Should contain registers base address and length.
>>>>>>>>> +- interrupts:    Should contain one entry, DMA controller interrupt.
>>>>>>>>> +- clocks:    Should contain one entry, DMA controller clock.
>>>>>>>>> +- resets :    Should contain one entry, DMA controller reset.
>>>>>>>>> +- #dma-cells:    Should be <1>. The cell represents DMA request select
>>>>>>>>> value
>>>>>>>>> +        for the peripheral. For more details consult the Tegra TRM's
>>>>>>>>> +        documentation, in particular AHB DMA channel control register
>>>>>>>>> +        REQ_SEL field.
>>>>>>>>
>>>>>>>> What about the TRIG_SEL field? Do we need to handle this here as well?
>>>>>>>>
>>>>>>>
>>>>>>> Actually, DMA transfer trigger isn't related a hardware description. It's
>>>>>>> up to
>>>>>>> software to decide what trigger to select. So it shouldn't be in the binding.
>>>>>>
>>>>>> I think it could be, if say a board wanted a GPIO to trigger a transfer.
>>>>>>
>>>>>>> And I think the same applies to requester... any objections?
>>>>>>
>>>>>> Well, the REQ_SEL should definitely be in the binding.
>>>>>>
>>>>>> Laxman, Stephen, what are your thoughts on the TRIG_SEL field? Looks
>>>>>> like we never bothered with it for the APB DMA and so maybe no ones uses
>>>>>> this.
>>>>>
>>>>> I don't think TRIG_SEL should be in the binding, at least at present. While
>>>>> TRIG_SEL certainly is something used to configure the transfer, I believe the
>>>>> semantics of the current DMA binding only cover DMA transfers that are initiated
>>>>> when SW desires, rather than being a combination of after SW programs the
>>>>> transfer plus some other HW event. So, we always use a default/hard-coded
>>>>> TRIG_SEL value. As such, there's no need for a TRIG_SEL value in DT. There's
>>>>> certainly no known use-case that requires a non-default TRIG_SEL value at
>>>>> present. We could add an extra #dma-cells value later if we find a use for it,
>>>>> and the semantics of that use-case make sense to add it to the DMA specifier,
>>>>> rather than some other separate higher-level property/driver/...
>>>>
>>>> Thank you for the comment. If we'd want to extend the binding further with the
>>>> trigger, how to differentiate trigger from the requester in a case of a single
>>>> #data-cell?
>>>>
>>>> Of course realistically a chance that the further extension would be needed is
>>>> very-very low, so we may defer the efforts to solve that question and for now
>>>> make driver aware of the potential #dma-cells extension.
>>>
>>> The request selector cell isn't optional, so is always present. If we later add
>>> an optional trig_sel cell, we'll either have:
>>>
>>> #dma-cells=<1>: req_sel
>>>
>>> or:
>>>
>>> #dma-cells=<2>: req_sel, trig_sel
>>
>> Why request sel. couldn't be optional? Could you please elaborate a bit more?

The documentation currently says it's mandatory, and DT bindings must be 
evolved in a backwards-compatible fashion.

>> I think possible options are:
>>
>> #dma-cells=<1>: req_sel
>> #dma-cells=<1>: trig_sel
> 
> With the above, how would you know that it is the req_sel or trig_sel
> that is specified?

Also, if req_sel were optional, then it'd be impossible to distinguish 
between those cases, so we can't design a binding like that. In general, 
when adding extra optional cells to an #xxx-cells style binding, then 
whenever cell N is present, all cells before cell N must be present even 
if optional.